Mobile communication systems have been developed with the aim to facilitate communication everywhere, with everyone and at any time. In recent years, mobile communication systems, and particularly cellular communication systems, experienced a huge increase, both in the number of users and in the quality and demands of services offered.
Commonly used and widely spread mobile communication systems such as the pan-European Global System for Mobile Communication (GSM) are cellular systems. A cellular system or network is characterized in that it is organized on a cell basis, wherein each cell comprises a base station whose radio coverage area defines the geographical spreading of this cell.
Since only a limited frequency band is available for an entire mobile communication network and each communication channel requires a certain bandwidth, it is essential to exploit the available frequency band as efficient as possible such that as many users as possible can be serviced in the network. Therefore, in a cellular network, the available frequencies are usually reused on a cell basis. This means that the same set of frequencies, i.e. the same frequency band, which is used in one cell are also used in another cell of the same system in order to increase the user capacity of the system. However, in this regard, there exists a drawback in that interferences between the communications of users in different cells may occur, when the same frequencies are used. Such interferences are desired to be avoided since the communication quality is deteriorated due to them. Thus, the same frequencies are to be reused only in cells which are spaced at a minimum distance from each other. This distance is usually called spatial frequency reuse D (see FIG. 1). However, the larger distance D is and, thus, the lower potential interferences are, the fewer users can be serviced in the system, i.e. the lower the capacity of the system is. That is the spatial frequency reuse D is desired to be as small as possible, in particular in view of an increasing number of users.
In FIG. 1, a cell structure of a cellular mobile communication system is depicted according to the hexagon model, i.e. each cell is illustrated as a hexagon, irrespective of its actual physical shape. The number in each cell denotes a certain set of frequencies, i.e. a certain frequency band, which is allocated to this cell. According to FIG. 1, three frequency bands 1, 2, 3 are exemplarily used and a cell using frequency band 1 is surrounded by cells using frequency bands 2 and 3. This results in a frequency reuse factor k being 3.
Within one cell communications are effected in so-called channels. In the GSM system, for example, the channel allocation comprises a segmentation of channels both in the frequency domain and in the time domain. Thus, by dividing the available frequencies in an uplink band (for communication between a mobile station and a base station) and a downlink band (for communication between a base station and a mobile station) a frequency division duplex (FDD) technique is implemented. Further, by dividing an uplink/downlink band in a time frame structure comprising time slots, a time division duplex (TDD) technique is implemented. Other communication systems may use one of these techniques, either FDD or TDD, or a combination of these.
In order to cope with the increasing requirements mentioned above in terms of number of users and demands of services, which are posed on mobile communications, mobile systems and networks of the third generation (3G) and even the fourth generation (4G) are under development and partly already in operation, e.g. the General Packet Radio Service (GPRS) and the Universal Mobile Telecommunication System (UMTS).
The current working assumption for a 4G cellular system in a high frequency bandwidth requirement amounts to 1 Gbps (Gigabits per second) in maximum data rate. To achieve reasonable multi-operator scenarios in view of suchlike requirements and with limited total bandwidth availability, the frequency reuse factor in the network must be low. Further, for a continuous coverage of the whole cell by its base station, pilot and broadcast channels must be receivable over the whole cell area, which may also result in overlaps with neighboring cells. However, such overlaps are adverse with respect to the aim of a smaller frequency reuse factor, since overlaps between cells using the same frequency bands would again result in deteriorating interferences.
Generally, an overlapping can be avoided or, at least, reduced by accordingly affecting the transmission powers of cells using the same frequency band. A method for channel allocation utilizing power restrictions is presented in U.S. Pat. No. 6,259,685. In this method the time-slotted transmissions of synchronized base stations are arranged in such a way that transmissions utilizing maximum power P do not occur at the same time t in cells sharing the same frequency band.
The principle of a time-slotted transmission power scheme according to the cited prior art solution is illustrated in FIG. 2. The figure shows the power restrictions of the base station for a situation of three neighboring cells, with P denoting the transmission power of the base station of the respective cell and t denoting the time. In a normal situation, the single timeslots are allocated to different terminals at different geographical locations.
This prior art method results in a kind of “breathing” in the cell coverage areas, which would in this case be the desired outcome producing the spatial overlap in the border zone between two cells, enabling camping of a mobile station on either cell.
However, there are shortcomings of the prior art solution according to FIG. 2 in that it is not specified, in which time slot or time slots pilot and/or broadcast channels are transmitted. That is, there is no regulation provided by this method on how to arrange transmissions of cell management information such as pilot and broadcast channels, both within a time frame structure of a cell as well as in relation to neighboring cells.
Hence, on the one hand, it is possible that pilot and broadcast channels of different cells using the same frequencies are transmitted at the same time instant or clashing time instants. Even though in this case, the transmissions would have different power levels, which reduces a risk of spatial overlapping, a mobile station receiving all of the three transmissions of cell management information would not be able to conduct correct measurements of the cell management information relating to neighboring cells. Thus, it is at least difficult for the mobile station to select the most suitable cell or base station to assign to.
On the other hand, in case pilot and/or broadcast channels are transmitted in time slots with a low transmission power P, an overlapping might be avoided or reduced, but a full and continuous coverage of the whole cell area may not be ensured. Thus, a mobile station being located in (the border zone of) the actual cell area may not be able to receive pilot and/or broadcast channels, whereby the mobile station does not obtain the necessary cell management information. Moreover, if the power of the pilot signal is not synchronized to the power limits and becomes variable, the “cell breathing” results in unwanted continuous handovers between base stations.
Thus, a solution to the above problems and drawbacks is desirable for a cellular communication network, in which frequency reuse possibilities are limited.